Bacterial leaching of copper and zinc without iron leaching

ABSTRACT

Thiobacillus ferrooxidans  and  Thiobacillus thiooxidans  play major roles in bacterial leaching of metals from ores and tailings through their oxidative action on ferrous iron (Fe 2+ ) and sulfide or sulfur (S 0 ). We have studied the effects of various inhibitors such as cyanide, azide and anions (phosphate, nitrate, chloride) on Fe 2+  or S 0  oxidation by resting cells, on Fe 2+  or S 0  as an energy source for growth and finally on leaching of Fe or Zn from a sample containing pyrite (FeS 2 ) and sphalerite (ZnS). All of these inhibitors inhibited Fe 2+  oxidation more strongly than S 0  oxidation and generally stopped the growth on Fe 2+  at lower concentrations than on S 0 . All three anions inhibited the leaching of Fe more strongly than Zn leaching, but potassium phosphate was the most selective. In the leaching experiments with  T. ferrooxidans  and  T. thiooxidans  either singly or combined, phosphate at 10-100 mM reduced the Fe leaching almost completely (by 90-over 95%) without affecting the Zn leaching (0-40%). Furthermore, the method can be used for recovery of other minerals from other sulfide ores.

[0001] The present invention relates generally to the field of bacterial leaching of minerals.

BACKGROUND OF THE INVENTION

[0002]Thiobacillus ferrooxidans and Thiobacillus thiooxidans play major roles in solubilization of metal from ores for economic recovery (Rossi, 1990, Biohydrometallurgy, McGraw-Hill: New York; Suzuki et al, 1990, Appl Environ Microbiol 56: 1620-1626; Rawlings and Silver, 1995, Bio/Technology 13: 773-778; Moffat, 1994, Science 264: 778-779). Both bacteria can oxidize sulfide or sulfur to sulfuric acid, but T. ferrooxidans can also oxidize ferrous iron to ferric iron as an energy source. Typically, carbon dioxide and ammonia are fixed to synthesize cell materials, using the energy for growth. Furthermore, the popular belief is that contributions from both of the oxidation activities are needed for effective bacterial leaching of metals from sulfide ores.

[0003] Most sulfide ores contain the iron mineral, pyrite (FeS₂), as well as valuable sulfide minerals, for example zinc sulfide (sphalerite, ZnS). As a consequence, bacterial leaching of metals is hampered by concurrent leaching of iron from pyrite, as the separation and removal of solubilized iron from the leached metal, for example, zinc, are difficult and expensive. However, it is important to note that pyrite solubilization involves both the oxidation of sulfide or sulfur and that of ferrous iron while sphalerite solubilization involves only the oxidation of sulfide or sulfur.

[0004] Clearly, it would be advantageous to be able to inhibit iron oxidation without inhibiting sulfur oxidation, to inhibit growth on iron without inhibiting growth on sulfur and to inhibit bacterial solubilization of iron without inhibiting bacterial solubilization of another mineral, for example, zinc and/or copper. As a result, it would be possible to solubilize sphalerite in the absence of pyrite solubilization, meaning that the added and expensive step of removal of the solubilized iron would not be necessary.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the invention to provide methods of inhibiting iron oxidation without inhibiting sulfur oxidation, inhibiting growth on iron without inhibiting growth on sulfur and inhibiting bacterial solubilization of iron without inhibiting bacterial solubilization of another mineral, for example, zinc and/or copper.

[0006] According to one aspect of the invention there is provided a method of leaching zinc from ore comprising:

[0007] providing a quantity of ore including pyrite (FeS₂) and sphalerite (ZnS);

[0008] providing iron-oxidizing or sulfur-oxidizing bacteria, said bacteria being capable of bacterial solubilization of pyrite and sphalerite to iron and zinc respectively;

[0009] providing a bacterial growth medium capable of supporting growth of the iron-oxidizing or sulfur-oxidizing bacteria;

[0010] supplementing the growth medium with an inhibitor that inhibits pyrite solubilization but does not inhibit sphalerite solubilization or inhibits sphalerite solubilization to a lesser extent than pyrite solubilization;

[0011] placing the ore and the iron-oxidizing or sulfur-oxidizing bacteria in the growth medium; and

[0012] incubating the bacteria and the ore under conditions permitting solubilization of the sphalerite to zinc. Thus, the bacteria act to solubilize the sphalerite to zinc without solubilization of the iron, thereby allowing recovery of the zinc from the ore without the added step of having to remove the iron.

[0013] The inhibitor may be an anion, for example: phosphate; nitrate; chloride; and mixtures thereof.

[0014] The inhibitor may be a respiratory inhibitor, for example: azide; cyanide; and mixtures thereof.

[0015] The inhibitor may be selected from the group consisting of phosphate, nitrate, chloride, azide, cyanide and combinations thereof.

[0016] The ore may be composed of tailings or mixtures of minerals.

[0017] Preferably, the iron or sulfur-oxidizing bacteria are selected from the group consisting of: Thiobacillus ferrooxidans; Thiobacillus thiooxidans; and a mixture thereof.

[0018] Preferably, the inhibitor is phosphate or chloride and the potassium salt is in the medium at a concentration of 10-100 mM.

[0019] Preferably, the bacterial growth medium comprises:

[0020] 0.4 g/L (NH₄)₂SO₄;

[0021] 0.1 g/L K₂HPO₄;

[0022] 0.4 g/L MgSO₄.7H₂O; and

[0023] 10-100 mM phosphate or chloride,

[0024] adjusted to pH 2.3 with H₂SO₄.

[0025] According to a second aspect of the invention, there is provided a growth media for iron-oxidizing or sulfur-oxidizing bacteria for bacterial leaching of a mineral from ore comprising:

[0026] 0.4 g/L (NH₄)₂SO₄;

[0027] 0.1 g/L K₂HPO₄;

[0028] 0.4 g/L MgSO₄.7H₂O; and

[0029] 10-100 mM phosphate or chloride,

[0030] adjusted to pH 2.3 with H₂SO₄.

[0031] According to a third aspect of the invention, there is provided a method of leaching a mineral from ore comprising:

[0032] providing a quantity of ore including chalcopyrite (CuFeS₂) or pyrite (FeS₂) or similar iron-containing minerals and sulfide mineral of a metal;

[0033] providing iron-oxidizing or sulfur-oxidizing bacteria, said bacteria being capable of bacterial solubilization of pyrite and the sulfide mineral to iron and the metal respectively;

[0034] providing a bacterial growth medium capable of supporting growth of the iron-oxidizing or sulfur-oxidizing bacteria;

[0035] supplementing the growth medium with an inhibitor that inhibits iron solubilization but does not inhibit solubilization of the sulfide mineral or inhibits solubilization of the sulfide mineral to a lesser extent than iron solubilization;

[0036] placing the ore and the iron-oxidizing or sulfur-oxidizing bacteria in the growth medium; and

[0037] incubating the ore and the bacteria under conditions permitting solubilization of the sulfide mineral to the metal. Thus, the above-described method can be used for bacterial solubilization of other sulfide minerals from pyrite-containing ores.

[0038] The ore may be selected from the group consisting of pyrite, sphalerite, chalcopyrite and mixtures thereof.

[0039] The metal may be zinc, copper or mixtures thereof.

[0040] The inhibitor may be an anion. The anion may be selected from the group consisting of phosphate, nitrate, chloride and mixtures thereof.

[0041] The inhibitor may stimulate solubilization of the sulfide mineral.

[0042] The solubilization of metals may occur at different times, permitting the separate recovery of each metal.

[0043] One embodiment of the invention will now be described in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Table 1 is a summary of the inhibition of Fe²⁺ oxidation and S⁰ oxidation by the various compounds.

[0045] Table 2 is a summary of the growth inhibition by the various compounds.

[0046] Table 3 is a summary of the effects of various anions on bioleaching of pyrite and sphalerite mixtures.

[0047] Table 4 is a summary of the effects of various anions and concentrations thereof in bioleaching of chalcopyrite and sphalerite mixture.

[0048] Table 5 is a summary of the effects of anions on bioleaching of a complex sulfide ore.

[0049]FIG. 1 is a graph of the effect of phosphate and chloride on the Fe²⁺ and S⁰ oxidation during the growth of T. ferrooxidans and T. thiooxidans respectively in Micro-oxymax experiments.

[0050]FIG. 2 is a bar graph representation of the inhibition of Fe solubilization by potassium phosphate in T. ferrooxidans bacterial leaching of tailings.

[0051]FIG. 3 is a bar graph of representation of the inhibition of Fe solubilization by potassium phosphate in T. thiooxidans bacterial leaching of tailings.

[0052]FIG. 4 is a bar graph representation of the inhibition of Fe solubilization by potassium phosphate in bacterial leaching of tailings with both T. ferrooxidans and T. thiooxidans.

[0053]FIG. 5 is a time course of extraction of copper and zinc from ore.

DETAILED DESCRIPTION

[0054] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned hereunder are incorporated herein by reference.

[0055] Herein, a study of iron and sulfur oxidation by Thiobacillus thiooxidans and Thiobacillus ferrooxidans in the presence of varying concentrations of various anions and respiratory inhibitors is described. Specifically, the anions used were phosphate, nitrate and chloride while the respiratory inhibitors used were cyanide and azide. That is, it is shown that: iron oxidation can be inhibited without inhibition of sulfur oxidation; bacterial growth on iron can be inhibited without inhibition of bacterial growth on sulfur; and bacterial solubilization of iron can be inhibited without inhibition of bacterial solubilization of sulfur. Thus, the method described herein can be utilized to recover a variety of minerals in iron-containing ores without the added step of separating the mineral of interest from iron solubilized from pyrite.

[0056] The invention will now be described by way of examples, although it is to be understood that the invention is not limited to these examples.

EXAMPLE I

[0057] Materials and Methods

[0058] In this embodiment of the invention, bacterial strains Thiobacillus ferrooxidans SM-4 and Thiobacillus thiooxidans SM-6 (FIGS. 1, 3 and 4 and Table 2) or SM-7 (Table 5) were used. Specifically, T. ferrooxidans was grown on Fe²⁺ or S⁰ while T. thiooxidans was grown only on sulfur. HP medium, described in Suzuki et al, 1990, Appl Environ Microbiol 56:1620-1626, consisting of: 0.4 g (NH₄)₂SO₄, 0.1 g K₂HPO₄, 0.4 g MgSO4.7H₂O per liter, adjusted to pH 2.3 with H₂SO₄ was used for growth of bacteria on Fe²⁺, for growth in a Micro-oxymax™ respirometer (both Fe²⁺ and S⁰ growth), and for mineral leaching experiments, as described below. For growth on Fe²⁺, 33.3 g FeSO₄.7H₂O per liter (pH 2.3, filter-sterilized) was added to the above-described HP medium. Cultures were grown in shake flasks (100 ml) with 10% inoculum (v/v) in 250 ml Erlenmeyer™ flasks at 25° C. at 120 rpm for 24 hours. It is of note that for large scale growth, medium volume and flask size were increased.

[0059] The medium used for growth on S⁰, unless otherwise indicated, was Starkey No. 1 medium: 0.3 g (NH₄)₂SO₄, 3.5 g KH₂PO₄, 0.5 g MgSO₄.7H₂O, 0.25 g CaCl₂, 18 mg FeSO₄.7H₂O per liter, adjusted to pH 2.3 with H₂SO₄. Elemental sulfur powder (sulfur precipitated, BDH Chemicals, Toronto), 2.5 g, was spread on the surface of 100 ml medium in 250 ml Erlenmeyer™ flasks after the addition of 5% (v/v) inoculum. Flasks were then covered with tissue paper and incubated at 25° C. for 4 days without shaking or stirring. For large scale growth, 1.0 liter medium in 2.8 liter Fernbach™ flasks supplemented with 25 g sulfur were used and the cultures were incubated at 28° C.

[0060] For the resting cell experiments, cells from large scale growth were filtered through Whatman™ No. 1 filter paper under suction before centrifugation at 8000× g. The cells were washed in 0.1 M β-alanine-H₂SO₄ (pH 2.3) and suspended in the same buffer to a concentration of 50 mg wet cells per ml.

[0061] The rate of ferrous iron and sulfur oxidation by the resting cells was determined by the oxygen consumption rate (nmol O₂/min) in a Gilson™ Oxygraph with a Clark™ oxygen electrode at 25° C. The reaction mixture in a total volume of 1.2 ml contained 10-100 μl cell suspension, 0.5 μmol FeSO₄ or 0.1 ml elemental sulfur suspension (32 mg in 500 ppm Tween-80™) and 0.1 M β-alanine-H₂SO₄ at pH 3.0 (standard condition) or varying inhibitory concentrations of salts at pH 3.0, as described below. Specifically, the effect of standard biological oxidation inhibitors, sodium azide and sodium cyanide in 0.1 M β-alanine-H₂SO₄ at pH 3.0 was studied.

[0062] Growth inhibition studies were carried out in a Micro-oxymax™ respirometer at Cominco Research in Trail, British Columbia, wherein the O₂ and CO₂ consumption (at 52 minute intervals) of cultures at 26° C. were monitored over time (45-50 hours). Each flask contained a total volume of 100 ml, comprised of 5 ml inoculum culture plus 95 ml HP medium with varying concentrations of inhibitors plus Fe²⁺ or S⁰ as growth substrate (33.3 g FeSO₄.7H₂O or 25 g sulfur per liter). In the S⁰ medium, HP medium was supplemented with 18 mg FeSO₄.7H₂O per liter. A growth curve was generated from the monitored O₂ and CO₂ consumption rates over time, as described below.

[0063] Bacterial leaching was performed in shake flask experiments with 250 ml Erlenmeyer™ flasks containing 95 ml of HP medium with and without inhibitors unless otherwise specified, 4 ml of inoculum (Fe²⁺-grown T. ferrooxidans, S⁰-grown T. thiooxidans or both) and 5 g of floatation tailings supplied by Cominco Research were used, as described below. The flasks were grown at 26° C. on a rotary shaker at 150 rpm for 14 days. At various time intervals, 5 ml samples were taken to determine the concentrations of dissolved zinc and iron in the leachate as well as the percentage of zinc and iron remaining in the precipitate (after filtration of the leachate).

EXAMPLE II

[0064] Inhibition of Oxidation

[0065] The oxidation of Fe²⁺ or S⁰ was differentially affected by various inhibitors as shown in Table 1. Specifically, 0.1 M potassium sulfate did not inhibit either Fe²⁺ or S⁰ oxidation while 0.3 M potassium sulfate inhibited Fe²⁺ oxidation (10%) to a lesser extent than S⁰ oxidation (30%). However, 0.1 M potassium phosphate inhibited Fe²⁺ oxidation (60%) but not S⁰ oxidation, while increasing the potassium phosphate concentration to 0.3 M inhibited Fe²⁺ oxidation to a similar extent as seen with 0.1 M potassium phosphate, (60%) although S⁰ oxidation was now inhibited somewhat (30%), as a result of the increased potassium phosphate concentration. Potassium chloride inhibited Fe²⁺ oxidation at a concentration of 0.1 M (30%) and at 0.3 M (40%) but inhibited S⁰ only at the 0.3 M concentration and to a lesser extent (10%). Potassium nitrate inhibited Fe²⁺ oxidation very strongly at both 0.1 M and 0.3 M (90% and 100% respectively) but inhibited S⁰ oxidation only at 0.3 M (85%) and not at 0.1 M (10%). Sodium azide inhibited Fe²⁺ oxidation strongly at either 10 μM (90%) or 50 μM (95%) while the difference in inhibition of S⁰ oxidation was much more pronounced at 50 μM than 10 μM (90% versus 35%). Furthermore, sodium cyanide inhibited Fe²⁺ oxidation strongly at 10 μM (90%) and 50 μM (95%) but inhibited S⁰ oxidation only moderately at 50 μM (25%). Thus, to summarize, potassium phosphate, potassium chloride and potassium nitrate all inhibited the oxidation of Fe²⁺ more strongly than the oxidation of S⁰. It is of note that phosphate and nitrate were more effective than chloride in this inhibition. Potassium sulfate, on the other hand, did not inhibit the Fe²⁺ oxidation even as much as S⁰ oxidation. Furthermore, the respiration inhibitors, sodium azide and sodium cyanide, also showed a stronger inhibition of Fe²⁺ oxidation.

EXAMPLE III

[0066] Inhibition of Growth

[0067] Growth inhibition results shown in Table 2 indicate the same differential inhibition as seen in Table 1, with the exception of potassium nitrate which inhibited both Fe²⁺ and S⁰ growth strongly. It is of note that the S⁰ oxidation in Table 1 was studied at pH 3, as we have previously shown that S⁰ oxidation by T. ferrooxidans is lower at pH 2.3. FIG. 1 shows that S⁰ oxidation was actually stimulated by 50 mM phosphate or chloride, while Fe²⁺ oxidation was inhibited.

EXAMPLE IV

[0068] Selective Leaching of Zinc

[0069] FIGS. 2 to 4 show the concentration of Zn and Fe solubilized after 14 days of bacterial leaching of flotation tailings. As can be seen, T. ferrooxidans (FIG. 2) solubilized more zinc than T. thiooxidans (FIG. 3), leaching nearly 100% (1.65 g/l) of the zinc, but also solubilized around 50% of total iron (2.75 g/l from the tailings plus 334 mg/l from 5% culture inoculum=3.08 g/l). However, as shown in FIG. 2, addition of potassium phosphate at concentrations of 25-100 mM drastically reduced the Fe solubilization with only 50% reduction in Zn leaching. It is of note that potassium chloride had little effect on Zn selectivity, but potassium nitrate reduced the Fe leaching similar to phosphate, although, in this instance, Zn leaching was also considerably inhibited. Furthermore, neither sodium azide nor sodium cyanide had a significant effect after 14 days although there was a slight inhibitory effect on both Fe and Zn leaching after two days. As can be seen in FIG. 3, potassium phosphate strongly inhibited Fe leaching with T thiooxidans at 10-100 mM. Furthermore, KNO₃ and KCl also reduced Fe leaching at high concentrations, albeit to a much smaller degree. As can be seen in FIG. 4, when both T. ferrooxidans and T. thiooxidans were present, potassium phosphate was again the most effective inhibitor of Fe leaching without inhibiting Zn leaching extensively. Thus, at 10 mM potassium phosphate, 70% Zn was solubilized with no Fe leaching.

EXAMPLE V

[0070] Stimulation of Copper Solubilization by Anions, Phosphate and Chloride

[0071] In this example, the effect of anions on metal solubilization from different mineral combinations was examined. Specifically, as shown in Table 3, bioleaching of a mixture of pyrite (FeS₂) and sphalerite (ZnS) by Thiobacillus ferrooxidans was affected by potassium phosphate exactly as expected based on the results obtained with flotation tailings, shown in FIG. 2. That is, potassium phosphate caused a strong inhibition of iron solubilization and moderate to no reduction of Zn solubilization. Potassium chloride showed a moderate inhibition of iron leaching, but stimulated zinc leaching.

[0072] When the study was extended to a mixture of chalcopyrite (CuFeS₂) and sphalerite, however, an additional effect of phosphate was observed as shown in Table 4. Specifically, potassium phosphate not only reduced iron oxidation but also increased the solubilization of Cu (from 9% to 20%). Extraction of Zn was slightly reduced by phosphate, but chloride stimulated Zn extraction. The bacterial leaching of a complex sulfide ore from the Flin Flon mine containing 4.9% Cu, 12.5% Zn, 30% Fe and 37.5% S, which is a mixture of pyrite, chalcopyrite and sphalerite, with small amounts of carbonates and pyrrhotite, showed stimulation of both Cu and Zn leaching by chloride as well as phosphate. In fact, the highest level of Cu leaching using this ore is nearly five times greater than previously reported values in the absence of additional anions (Lizama and Suzuki, 1988, Biotechnol Bioeng 32: 110-116). The stimulation was likely caused by the increased sulfur oxidation under these conditions (FIG. 1) to sulfuric acid, since whenever high concentrations of copper and zinc were leached, the pH of the leachate was very low (pH 1.5 to 2.0) while the pH of chemical control without bacteria remained at 3 to 4. Additionally, as shown in FIG. 5, the time course leaching of Cu and Zn is not identical. As can be seen in FIG. 5, copper solubilization may be largely finished prior to zinc leaching, suggesting that copper may be recovered first, prior to proceeding to zinc leaching and recovery. That is, copper and zinc can be recovered from a complex ore without the need for separation simply by replacing the media following completion of copper leaching because of this lag time.

EXAMPLE VI

[0073] Discussion

[0074] The effect of inorganic anions on T. ferrooxidans is known to be highly complex (Ingledew, 1982, Biochim Biophys Acta 683:89-117). Sulfate was considered essential for iron oxidation (Lazaroff, 1963, J Bacteriol 85:78-83) and replacement of sulfate with chloride or nitrate resulted in inhibition of iron oxidation (Lazaroff, 1963; Razzell and Trussell, 1963, J Bacteriol 85:595-603). However, it was shown that this sulfate requirement could be satisfied with selenate, that is, SeO₄ ²⁻ rather than SO₄ ²⁻ (Lazaroff, 1977, J Gen Microbiol 101:85-91). Sulfate is believed to affect an electron transfer component involved in the Fe²⁺ oxidation (Fry et al, 1986, Arch Biochem Biophys 246:650-654). It is of note that these studies centered on the sulfate requirement rather than inhibition by anions. However, it is hypothesized in Ingledew, 1982, that sulfate may have a role in lowering the redox potential of Fe²⁺/Fe³⁺ couple, making it more reducing and the oxidation of sulfur by T. ferrooxidans is also sensitive to ions (Razzell et al, 1963).

[0075] It is remarkable that the inhibition by various anions of Fe²⁺ oxidation, of growth on Fe²⁺ and of Fe-leaching follows the same pattern. The effect of phosphate on the selective leaching of Zn with nearly total inhibition of Fe leaching was particularly impressive. Since phosphate can react with both ferrous and ferric iron, the inhibition by phosphate may be different from simple anion inhibition. It could have a specific effect on Fe leaching from pyrite by T. ferrooxidans possibly by binding Fe on the surface of pyrite or cells.

[0076] It is of note that the concentration of potassium phosphate even at 10 mM is much higher than those required for growth of bacteria (McCready et al, 1986, Hydrometallurgy 17:61-71; Lizama and Suzuki, 1988, Biotecnol Bioeng 32:110-116)., which is 0.1 mM-0.57 mM, and might have some effect on the chemical (galvanic) interaction between pyrite and sphalerite, but since bacteria always stimulate the Zn solubilization beyond the chemical rate (Lizama and Suzuki, 1991, Can J Microbiol 37:304-311) and potassium nitrate can also reduce Fe solubilization, as discussed above, the effect of phosphate on these bacterial activities is the most likely explanation. That is, as discussed above and as shown in FIGS. 2 to 4, the addition of potassium phosphate strongly inhibited iron leaching by T. thiooxidans and T. ferrooxidans without inhibiting zinc leaching extensively.

[0077] Furthermore, as discussed above, the bacterial leaching of copper from chalcopyrite in the presence of sphalerite is stimulated by the addition of phosphate. Chloride stimulates zinc solubilization from sphalerite. Phosphate and chloride however both stimulate copper solubilization in a complex sulfide ore consisting of chalcopyrite, sphalerite and pyrite.

[0078] These stimulatory effects are in agreement with the observed stimulation of sulfur oxidation (FIG. 1) by 50 mM phosphate and chloride. When the sulfide portion of metal sulfides is oxidized to sulfate, the resulting metal sulfates are often soluble.

[0079] To summarize, it has been shown that iron oxidation can be inhibited without inhibition of sulfur oxidation, that growth on iron can be inhibited without inhibition of growth on sulfur and that bacterial solubilization of iron can be inhibited without inhibition of bacterial solubilization of sulfur or sometimes even with the activation of sulfur solubilization. As a result, the above-described method can be used to solubilize sulfide minerals within pyrite-containing ores in the absence of bacterial solubilization of iron. This in turn eliminates the expensive step of removing solubilized iron from the leachate, thereby making the process of bacterial solubilization of minerals from pyrite much more cost effective.

[0080] In other embodiments of the invention, other iron or sulfur-oxidizing bacteria may be used.

[0081] In yet other embodiments of the invention, leaching of other minerals may be done in the absence of iron leaching, provided of course that the other minerals are present in a sulfur-containing ore. These sulfide minerals may include for example sphalerite, chalcopyrite, covellite (CuS), chalcocite (Cu₂S) molybdenite (MOS₂), galena (PbS), stibnite (Sb₂S₃) argentite (Ag₂S), millerite (NiS), pentiandite ((Ni or Co)Fe)₉S₈, and mixtures thereof. As will be appreciated by one skilled in the art, this list is for illustrative purposes and is by no means intended to be exhaustive.

[0082] Since various modifications can be made in our invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense. 

1. A method of leaching zinc from ore comprising: providing a quantity of ore including pyrite (FeS₂) and sphalerite (ZnS); providing iron-oxidizing or sulfur-oxidizing bacteria, said bacteria being capable of bacterial solubilization of pyrite and sphalerite to iron and zinc respectively; providing a bacterial growth medium capable of supporting growth of the iron-oxidizing or sulfur-oxidizing bacteria; supplementing the growth medium with an inhibitor that inhibits pyrite solubilization but does not inhibit sphalerite solubilization or inhibits sphalerite solubilization to a lesser extent than pyrite solubilization; placing the ore and the iron-oxidizing or sulfur-oxidizing bacteria in the growth medium; and incubating the bacteria and the ore under conditions permitting solubilization of the sphalerite to zinc.
 2. The method according to claim 1 wherein the inhibitor is an anion.
 3. The method according to claim 2 wherein the anion is selected from the group consisting of: phosphate; nitrate; chloride; and mixtures thereof.
 4. The method according to claim 1 wherein the inhibitor is a respiratory inhibitor.
 5. The method according to claim 4 wherein the respiratory inhibitor is selected from the group consisting of: azide; cyanide; and mixtures thereof.
 6. The method according to claim 1 wherein the inhibitor is selected from the group consisting of phosphate, nitrate, chloride, azide, cyanide and combinations thereof.
 7. The method according to claim 1 wherein the ore is composed of tailings.
 8. The method according to claim 1 wherein the iron or sulfur-oxidizing bacteria are selected from the group consisting of: Thiobacillus ferrooxidans; Thiobacillus thiooxidans; and a mixture thereof.
 9. The method according to claim 1 wherein the inhibitor is phosphate or chloride.
 10. The method according to claim 9 wherein the phosphate or chloride is in the bacterial growth medium at a concentration of 10-100 mM.
 11. The method according to claim 9 wherein the phosphate or chloride is a potassium salt.
 12. The method according to claim 11 wherein the potassium salt is in the bacterial growth medium at a concentration of 10-100 mM.
 13. The method according to claim 1 wherein the bacterial growth medium comprises: 0.4 g/L (NH₄)₂SO₄; 0.1 g/L K₂HPO₄; 0.4 g/L MgSO₄.7H₂O; and 10-100 mM phosphate or chloride, adjusted to pH 2.3 with H₂SO₄.
 14. A growth media for iron-oxidizing or sulfur-oxidizing bacteria for bacterial leaching of a mineral from ore without iron leaching comprising: 0.4 g/L (NH₄)₂SO₄; 0.1 g/L K₂HPO₄; 0.4 g/L MgSO₄.7H₂O; and 10-100 mM phosphate or chloride, adjusted to pH 2.3 with H₂SO₄.
 15. A method of leaching a mineral from ore comprising: providing a quantity of ore including an iron ore and a sulfide mineral of a metal; providing iron-oxidizing or sulfur-oxidizing bacteria, said bacteria being capable of bacterial solubilization of the iron ore and the sulfide mineral to iron and the metal respectively; providing a bacterial growth medium capable of supporting growth of the iron or sulfur-oxidizing bacteria; supplementing the growth medium with an inhibitor that inhibits iron solubilization but does not inhibit solubilization of the sulfide mineral or inhibits solubilization of the sulfide mineral to a lesser extent than iron solubilization; placing the ore and the iron or sulfur-oxidizing bacteria in the growth medium; and incubating the ore and the bacteria under conditions permitting solubilization of the sulfide mineral to the metal.
 16. The method according to claim 15 wherein the ore comprises pyrite, sphalerite, chalcopyrite, covellite (CuS), chalcocite (Cu₂S), millerite (NiS), pentlandite ((Ni or Co)Fe)₉S₈ or mixtures thereof.
 17. The method according to claim 15 wherein the metal is selected from the group consisting of zinc, copper and mixtures thereof.
 18. The method according to claim 15 wherein the inhibitor is selected from the group consisting of phosphate, nitrate, chloride and mixtures thereof.
 19. The method according to claim 15 wherein the inhibitor stimulates solubilization of the sulfide mineral.
 20. The method according to claim 15 wherein the solubilization of metals occurs at different times, permitting the separate recovery of each metal. 